1. Field of the Invention
The present invention relates to a magnetic material. More particularly, it relates to a ferrite type magnetic material having a low magnetic loss when used in a high frequency band.
2. Description of the Related Art
As electronic technique has been developed recent years, electronic apparatuses have not only become compact and of high density but have become also begun to be used at a higher frequency for improving their efficiencies. For example, a magnetic material used in a magnetic core of a converter in a switching power supply, an inductance component and the like have been required to be compact as well as to be applied for use at a higher frequency. Such a material which has a low magnetic loss when used in a high frequency band has been desired so as to prevent heat generation caused especially in a compact apparatus.
As an example of the prior art, magnetic materials used in a magnetic core and the like will now be described. The magnetic materials are categorized into two major types, metal type materials and oxide ferrite type materials. The metal type materials have advantages of having a high saturation magnetic flux density and a high magnetic permeability. But they have a disadvantage that magnetic loss due to an eddy current is increased when used in the high frequency band since they have a low electric resistivity of about 10.sup.-6 to 10.sup.-4 .OMEGA..multidot.cm. As this disadvantage can be overcome by making the magnetic material thin, a metal is shaped into a thin foil and rolled up together with an insulator placed thereon. However, the lower limit of the thickness of the metal type materials is about 10 .mu.m, which is also the lower limit in the above-mentioned reduction of an eddy current. Moreover, the metal type materials are disadvantageously difficult to be formed into complicated shapes and expensive. Thus, such materials can not be used in a frequency bend over 100 kHz.
Alternatively, the ferrite type materials have a low saturation magnetic flux density of about a half of that of the metal type materials. However, the electric resistivity thereof is much higher than that of the metal type materials: for example, a generally used MnZn type ferrite has an electric resistivity of about 1 .OMEGA..multidot.cm. Moreover, the electric resistivity can be further increased up to about 10 to several hundreds .OMEGA..multidot.cm by adding CaO, SiO.sub.2 and the like. As a result, the magnetic loss due to an eddy current is small and such materials can be used in a relatively high frequency band. In addition, the ferrite type materials have advantages that they can be easily formed into complicated shapes and are inexpensive. Therefore, the ferrite type materials have been generally used in a magnetic core of a converter in a power supply that is used at a switching frequency of, for example, 100 kHz or more and 500 kHz or less.
However, the ferrite type materials can not be used at a frequency of 500 kMz or more because the magnetic loss due to an eddy current is increased at such a high frequency.
When the temperature coefficient of the magnetic loss is positive at around room temperature, the magnetic core emits heat due to the magnetic loss while in use, resulting in a raised temperature. As the temperature rises, the magnetic loss is further increased to make the magnetic core emit more heat. This cycle is repeated, and as a result, a thermorunaway may occur. Therefore, the ferrite type materials need to have a temperature characteristic that the temperature coefficient of the magnetic loss is negative at around room temperature and the magnetic loss is minimized at a temperature where the ferrite type materials are actually used, that is, from room temperature to 80.degree. C. At present, there is no MnZn type ferrite type material that has a sufficiently low magnetic loss when used in a high frequency band. For example, a conventional MnZn type ferrite has a magnetic loss of 1000 kW/m.sup.3 or more when used at a frequency of 1 MHz. Moreover, materials which have a comparatively low magnetic loss when used in the high frequency band generally have a bottom temperature of a magnetic loss around room temperature and the temperature coefficient thereof is positive. Thus, a thermorunaway can be easily caused. On the contrary, materials which have a bottom temperature of a magnetic loss over room temperature when used in the high frequency band have a very large magnetic loss. Thus, a material having an extremely low magnetic loss when used in the high frequency band and having a bottom temperature of a magnetic loss sufficiently higher than room temperature has not been provided at the present time. The temperature at which a material has a minimum magnetic loss is referred to as the "bottom temperature of a magnetic loss" hereinafter.
Among magnetic characteristics of a ferrite, characteristics such as the saturation magnetic flux density, the Curie temperature and the bottom temperature of the magnetic loss generally depend upon the composition of main components. Characteristics each as the magnetic permeability, a residual magnetic flux density, a coercive force and the magnetic loss depend also upon the composition of main components but mainly depend upon the fine structure of the ferrite. Any MnZn type ferrite having a low magnetic loss when used in the high frequency band is required to be high in saturation magnetic flux density, the Curie temperature, the bottom temperature of a magnetic loss and the magnetic permeability (K. Okutani, J. Jpn. Soc. Powder and Powder Metallurgy, 34, (5), p. 191 (1987)).
For example, the saturation magnetic flux density of the MnZn type ferrite is increased when a specific amount of ZnO and a large amount of Fe.sub.2 O.sub.3 are contained therein. However, when the amount of ZnO is too large, the Curie temperature is disadvantageously decreased. In addition, it is known that the magnetic permeability and the electric resistivity is decreased when the amount of Fe.sub.2 O.sub.3 is too large. The electric resistivity is decreased because the Fe.sub.2 O.sub.3 exceeding 50 mol % is changed into FeO, thereby causing an electron hopping between Fe.sup.2+ and Fe.sup.3+. Therefore, as the amount of Fe.sub.2 O.sub.3 exceeds 50 mol %, the electric resistivity is decreased. This decrease in the electric resistivity is lowered to some extent by using an appropriate additive or the like. But the thus decreased electric resistivity is still large in comparison with that of a conductive material with a main component originally having a high electric resistivity and comprising an appropriate additive. This decrease of the electric resistivity causes an increase of the magnetic loss due to an eddy current. Therefore, it has been believed that a composition including a large amount of Fe.sub.2 O.sub.3 can not be used in the high frequency band.
For example, Japanese Laid-Open Patent Publication No. 61-101458 discloses a MnZn type ferrite comprising 52 to 58 mol % of Fe.sub.2 O.sub.3, 7 mol % or less of ZnO and 35 to 48 mol % of MnO as main components, and 0.01 to 0.2 wt % of CaO and optionally 0.1 wt % or less of SiO.sub.2 as sub-components. This ferrite can be used at a frequency of 100 kHz.
The bottom temperature of a magnetic loss has been considered to depend upon a temperature characteristic of the magnetic permeability. When the MnZn type ferrite is measured for its magnetic permeability against a certain temperature, two peaks are generally obtained. One is a peak at the Curie temperature, which is called a primary peek due to the Hopkinson effect. The other is a peak at around room temperature, which is called a secondary peak. At the temperature of this secondary peak, the crystallomagnetic anisotropic coefficient K.sub.1 is 0. It has been believed that the magnetic loss is a minimal at this temperature.
The crystallomagnetic anisotropic coefficient K.sub.1 is linearly increased as the temperature rises. When K.sub.1 is negative at room temperature, K.sub.1 is 0 over room temperature. When a composition having such a coefficient is used, the bottom temperature of a magnetic loss can be set over room temperature. K. Ohta, J. Phys. Soc. Japan, 18, p. 684 (1963) describes a change of K.sub.1 obtained by varying a composition ratio of the, main components of the Mn/Zn type ferrite. K.sub.1 is slightly affected by the Mn/Zn ratio. K.sub.1 is positive and maximum at room temperature when the ferrite contains about 60 mol % of Fe.sub.2 O.sub.3. K.sub.1 is decreased when the ferrite contains less or more Fe.sub.2 O.sub.3. When the ferrite contains about 55 mol % or less of Fe.sub.2 O.sub.3, or about 61 mol % or more, K.sub.1 is less than 0 at room temperature. Accordingly, the bottom temperature of the magnetic loss can be controlled to be over room temperature when the composition of the ferrite is within the above two ranges. However, when the ferrite contains too much Fe.sub.2 O.sub.3, the magnetic permeability and the electric resistivity are decreased as mentioned above.
Because of the above-mentioned reasons, it has been believed that the most appropriate composition of the main components of the MnZn type ferrite having a low magnetic loss and a high bottom temperature of a magnetic loss is about 53 to 54 mol % of Fe.sub.2 O.sub.3, about 9 to 12 mol % of ZnO and the rest of MnO (S. Okamoto, et al., Electronic Ceramics, 16, p. 44 (winter, 1985)). Therefore, most of the ferrites with a low magnetic loss, which have actually been developed up to the present, have the above-mentioned composition. As an approach to decrease the magnetic loss, a method using compositions within or around the above-mentioned range for modifying the kind of additives or the fine structure of the ferrite has been mainly studied. The best ferrite obtained from such an approach has a low magnetic loss of about 500 kW/m.sup.3 when used at a frequency of 1 MHz.
For example, Japanese Laid-Open Patent Publication No. 1-224265 discloses a MnZn type ferrite comprising 52.2 to 55.4 mol % of Fe.sub.2 O.sub.3, 4 to 13.5 mol % of ZnO and 31.1 to 43.8 mol % of MnO as main components, and further comprises at least one sub-component selected from 0.05 to 0.2 wt % of CaO, 0.015 to 0.027 wt % of SiO.sub.2, 0.05 to 0.6 wt % of TiO.sub.2 and 0.01 to 0.2 wt % of Ta.sub.2 O.sub.5. This ferrite is sintered from powders having a particle size of 5 .mu.m or less, and shows a magnetic loss of 300 to 400 kW/m.sup.3 at 1 MHz.multidot.50 mT.